Background: Chronic kidney disease (CKD) encompasses a long-term decrease in function of the kidneys which afflicts 11.5% in American adults aged 20 and over, which is more than 20 million people. CKD can progress to kidney failure or end-stage renal disease (ESRD). At the end of 2008, 547,982 patients were receiving ESRD therapy, which includes hemodialysis or kidney transplant. Patient and renal graft survival rates have increased over the past five decades, but long-term survival of grafts is still an issue. Renal biopsy is the gold standard for diagnosis of kidney health, but is an invasive procedure, can not be used frequently, and can cause complications. Noninvasive indicators of kidney disease including levels of serum creatinine, glomerular filtration rate, and classical medical imaging can provide certain insights into kidney disease state. Elasticity imaging methods are emerging as a way to discriminate healthy versus diseased tissue based on the elasticity or viscosity of the tissue. When the structure of tissue changes due to disease processes, the function and viscoelastic properties also change. We propose the use of a quantitative, noninvasive method called shearwave dispersion ultrasound vibrometry (SDUV) to measure these quantities for assessment of renal health. Method: SDUV uses focused ultrasound to push the tissue and create shear waves. Ultrasound-based methods are used to detect the propagation of the shear waves through the tissue. The propagation speed of the shear waves varies with frequency, a phenomenon known as dispersion. This shear wave speed dispersion can be used to evaluate the shear elasticity and viscosity of the tissue. Studies in many different types of tissue including liver, kidney, skeletal muscle, prostate, heart and arteries have shown that SDUV can be a useful method for quantitatively evaluating the viscoelastic properties of tissue. SDUV measurements are noninvasive, fast, and localized to a small region of tissue. This noninvasive measurement technique can be suitable as a first-line screening tool to reduce the numbers of biopsies. Additionally, it can be used as a quantitative indicator for frequent assessment and monitoring of patients undergoing treatment regimens to track progress. SDUV techniques developed in this project could be implemented on modern ultrasound scanners with software modifications for widespread clinical translation. Aims: 1. Develop analytic methods and optimize implementation of SDUV for viscoelastic characterization of the native and transplanted human kidney. 2. Study the relationships between SDUV measurements of the viscoelastic properties of native kidneys of healthy kidney donors and patients with chronic kidney disease. 3. Study the relationship between SDUV measurements of viscoelastic material properties with clinical measures for assessment of kidney transplant rejection.